i held up the kung fujas to other ski's at a shop, and the 179 was longer than a pair of 183's by a bit, every ski around 180 that i compared them to was shorter than the kung fujas by a bit, id guess its really like a 185 or something. so a 189 would really be like a 195 which is huge. they are very stable ski's so i think the 179 would be more than enough, but if your only doing slow stuff in the park 189's might not be bad, and it would be pretty cool, just dont ever expect to 270 on to anything. but in 189 they would be awesome for jumps and slow steezy stuff. and a 189 would be awesome in powder and stuff.
i guess it depends if you want to use them more for park or all mtn, 179 is more than long enough for park, and even all mountain, but if you know your gonna be doing more powder and stuff 189 might be better